In U S. Pat. No. 3,471,343 a process for producing valve seat rings by powder metallurgy techniques is described. In the patented process, a powder mixture containing 0.8% to 1.5% by weight graphite, 1.0% to 4% by weight lead, 0.5% to 5% by weight nickel, 1.2% to 1.8% by weight molybdenum, 9.6% to 14.4% by weight cobalt, and the remainder iron, is pressed into valve seat rings for internal combustion engines and the like at a pressing force between about 40 and 60 KN/cm.sup.2 (1000 newtons per square centimeter). The pressed rings are then sintered in a neutral atmosphere at a temperature of about 1100.degree. C. to 1200.degree. C. and finally compressed at a pressing force above 120 KN/cm.sup.2 and, if required, heat-treated. The final compression may be a hot or cold compression and may be carried out by heating the valve seat rings after compression for 15 minutes to a temperature above the AC.sub.3 point, then cooling them and tempering them for 30 minutes at a temperature of about 600.degree. C.
Valve seat rings made by the process described in the aforesaid U.S. Pat. No. 3,471,343 have increased hot strength and are used for internal combustion engine exhaust valves. The hardness of the material is about 320 Brinell at room temperature and 205 Brinell at a temperature of 600.degree. C. However, the valve seat rings of the type described above, when manufactured in accordance with prior-art techniques, have a limited useful life when used in engines operated with lead-free fuel.